digital media dr. jim rowan itec 2110 video
TRANSCRIPT
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Digital Media
Dr. Jim Rowan
ITEC 2110
Video
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http://en.wikipedia.org/wiki/Interlacehttp://en.wikipedia.org/wiki/Progressive_scanhttp://en.wikipedia.org/wiki/Telecine
http://en.wikipedia.org/wiki/Digital_terrestrial_televisionhttp://en.wikipedia.org/wiki/ATSC_Standards
http://en.wikipedia.org/wiki/NTSChttp://en.wikipedia.org/wiki/PALhttp://en.wikipedia.org/wiki/SECAM
Lecture URLs
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http://www.bealecorner.com/gl1/res/gl1resint.jpg Test Pattern
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Next Several Lectures
Film & TV & Video & Animation
Issues that arise from conversion
Analog vs Digital
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TV Broadcast…Digital replaces Analog
Why Digital Broadcast?reduced spectrum usegreater capacity
multiple programs on one freqbetter quality pictureHDTVcan use compression
allows multiple HD signals on one freq.allows user interaction
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TV Broadcast…
Standards
Analog (old school)NTSC (north America)PAL (western europe)SECAM (eastern europe)
DigitalATSC (see map)
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TV Broadcast…Difference with poor reception
Analog…as signal gets weakerimage gets less distinct“ghosts (white shadows) appear”gracefully degrades
Digital…with digital, you either have signalor you don’t have signal so…lose signal
everything goes blackaudio stopsungraceful degrading
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Moving Pictures:Video & Animation
• In this class:– Video
• shot with a camera • captures images from the world• then play them back
– Animation • create frames individually• using inkscape and blender• play them back
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Video & Animation
• In this class:– Video
• shot with a camera • captures images from the world• then play them back
– Animation • create frames individually• using inkscape and blender• play them back
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Video (and Film)
• Works because of persistence of vision– human perception causes still images played in rapid
succession to fuse into motion– http://en.wikipedia.org/wiki/Frame_rate
• Fusion frequency– ~ 40 frames per second– depends on the brightness of the image relative to the
viewing environment
• Less than that– first flickering– then individual images appear losing the illusion of motion
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Film how it works
• Plays at 24 frames per second– Show the image– Block the light to make it dark– Move to the new image– Allow the light through to show the new
image– Without “blacking out” the change from one
image to the next the image would be blurred
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Video & TV
• Two versions– Interlaced
• Rising from a TV legacy
– Progressive scan• Rising from a computer legacy
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Interlaced
– Captured (and displayed) as “fields”– First the odd numbered lines are captured (or displayed)– Then the even numbered lines are captured (or
displayed)– This reduced the bandwidth needed to transmit images
that moved for early TV• The glowing phosphor of the CRT stayed glowing for a while
after the electron beam was turned off
• Allowing the other field to be drawn and complete the TV image
http://en.wikipedia.org/wiki/Interlace
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http://www.infocellar.com/television/files/interlace1.gif
Interlaced fieldsRaster scan
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Interleaved scanhttp://www.geniusdv.com/weblog/archives/2007/08/14/interlaced_vs_progressive.gif
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Interlaced problem:
• Rapid motion resulted in the “comb effect”
http://en.wikipedia.org/wiki/File:Interlaced_video_frame_%28car_wheel%29.jpg
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Interlace problem:
the center-column
images are interlaced
the left-column
images are
progressive scan
the right-column
images use line doublers
bottom images are anti-aliased
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Progressive scanhttp://www.geniusdv.com/weblog/archives/2007/08/14/interlaced_vs_progressive.gif
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Progressive scanhttp://en.wikipedia.org/wiki/Progressive_scan
• Each line on the screen is painted one after the other from top to bottom
• Electronics are faster now so interlacing is not required
• If captured progressively, then the playback is straight forward
• If captured as interlaced fields, playing them back progressively is problematic
• disadvantage of progressive scan is that it requires higher bandwidth than interlaced video that has the same frame size
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Video… it’s bigHow do you deal with it?
• Playback degradation• Compression
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Video… it’s bigHandling with Playback
• Transport or playback not fast enough to keep up with the story?– something’s got to give– there’s too much data to either transport or display
• Some players just freeze the image and halt the audio– this kills the ability to tell the story
• Some players (like quicktime) make attempts to “degrade gracefully”
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Video: Graceful degradation
• Graceful degrading allows the story to continue• Some players drop frames
– first showing as a “slide show” while continuing to play the audio– then holding the last image while continuing to play the audio stream– this effectively loses the illusion of motion but continues the “story” as an
audio stream
• Some play lower resolution images while remaining synched to the audio stream– this continues the illusion of motion (at a lower resolution) and continues
the “story” with the audio stream
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Video is big so: reduce its size using compression
• On the capture side– Digitization & compression can be carried
out by hardware to be fast– Can be done in the camera (hardware)– Can be done in the computer (software)
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hardware vs software compression
• Hardware compression... user has no control over it... it is hardwired– It can be in the camera– It can be in the video card
• Software conversion... is computationally expensive... it’s a slow process– Provides for the most flexibility since it can be
changed– Can use different software coder-decoders (codec),
picking and choosing what fits your needs better
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Compression in the camera:hardware compression
• Our cameras?– Mini DV format– Compress each captured image into a jpeg image
• This is called intra-frame compression
– Present a digital stream of bits to the computer over a firewire connection
• With compression you get artifacts
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with software compression…
• Analog is presented to the computer through a video capture card
• Compression is done (usually) in the video capture card
• Allows for a really small camera because the work (the compression and the analog to digital conversion) is done elsewhere
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More aboutAnalog vs Digital
• An analog signal to the computer is susceptible to noise corruption
• Digital signal is not• What’s the big deal?• Consider compressing a video of a wall
painted a solid color– Analog noise will cause small fluctuations from
pixel to pixel– RLE can’t compress it because each pixel is a bit
different
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Comparing cameras iSight to MiniDV
• iSight (or a webcam) is built into the Macs in this room– Presents an analog signal to the computer– Subject to analog noise
• The cameras we can check out from the library are Mini DV format and record on tape– Presents a digital signal to the computer
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iMovie
video capture card
computer
miniDV
compression
compression
webCam
analog signal
digital signal
Our video cameras compress using jpeg
the scene
!!!NOISE!!!
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iMovie
video capture card
computer
miniDV
compression
compression
webCam
analog signal
digital signal
640 x 480 = 307,200307,200 can be represented by < 24 bits, call it 3 bytesRLE: 307,200 (3bytes) + RGB (3 bytes) ~ 6 bytes
Our video cameras compress using jpeg
the scene
640 x 480 = 307,200 bytesNoise makes each pixel a little differentRLE: 307,200 bytes x RGB (3bytes) = 921600 bytes
!!!NOISE!!!
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• Converting TV to Video is problematic– Interlacing
• comb effect
Next:
• Converting Film to Video is problematic– Matching 24 frames to 30 frames
• Telecine problem
We’ve seen…
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Film to Video
• Problematic– video is 30 frames per second– film is 24 frames per second
• How do you make 30 frames from 24?
• One way: The 3-2 pull down…
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Film to interlaced video: http://en.wikipedia.org/wiki/Telecine
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